http://wccftech.com/intel-kaby-lake-core-i7-7700k-cpu-leaked/
Assuming that is correct, which I certainly can't verify, it looks like the high end Kaby Lake CPU would be the i7-7700K which would be a 3.6GHz base clock 14nm CPU. The i7-6700K (Skylake) is a 4.0GHz base clock 14nm CPU. Both can run on Z170 based motherboards, but Kaby Lake should also see the release of the 200 series chipsets which are supposed to offer 24 PCIe 3.0 lanes whereas Z170 offers 20 PCIe 3.0 lanes.
IMO, if you intend to use a single GPU and at most an M.2 SSD (NVMe PCIe 3.0 x4) then you only need 20 PCIe 3.0 lanes to begin with. Everything else can connect via SATA or via the PCIe lanes from the Z170 chipset rather than the CPU itself.
The 6700K should be a capable chip for a very long time, and it would take a fairly sizeable IPC bump for the Kaby Lake 7700K to outpace the 6700K given the 400MHz base clock difference. Overclocking is a separate story; obviously if both are at the same speed then the Kaby Lake is a couple percent higher, or whatever the IPC gain is of this generation (we can't know).
As said above, you can always wait for new hardware. The cheap solution is to get a better CPU to fit your board and upgrade your RAM. If you lack an SSD, then absolutely add one of those. A 2500K with some decent overclock, coupled with 16GB RAM, and a good SSD should be satisfactory and should cost far less than a Z170 board, a 6700K, and DDR4.